Hi Gregory,
I built and live in a loadbearing two story cob house in Victoria BC (west coast) - high seismic area. It is code approved and permitted (first legal 2 storey seismacally engineered load bearing cob home in North America). I've also helped four other families do the same. There is no one way to build a cob house... but I would recommend the following.
Wall thickness is determined based on the compressive load it has to carry as well as the thermal performance you need to gain. (For thermal performance we have done some intense research found here under the "technical science report"
http://ecosenseliving.wordpress.com/research/ ). Cob is stronger as a monolithic unit then with things embedded in it. If you are building in a seismic area, then every 2 feet an engineer will want to see a horizontal layer of some form of grid-like material (geotextile or welded wire fencing)... if this is the case then you can build vertically... if you don't have to use this horizontal layer then the sloped walls are best.
As for detailing air gaps - if you detail the building properly, then it can be an airtight building. In BC, the Home Protection Office provides classes and training for builders and building officials... they classify cob as the most air tight building system - but if your connections at poles and roofsare not detailed well then its not. A word of warning about doing a load bearing structure with poles and infilling with cob... Gernot Minke has a section in his book Building with Earth where he warns about using two materials with different flexing properties adjacent to each other. Wood flexes under loads and wind shear whil cob does not. Eventually at these connections you will see failure. The route around this is to never have your cob come into contact with the supporting posts, something we are seeing adhered to more and more. Also some great earthen standards out there including the ASTM2392 which discusses some do's and don'ts.
Good luck and enjoy... it will be the healthiest building you will ever live in (in MHO).